Morphology of Rigor-shortened Bovine Muscle and the Effect of Trypsin on Pre- and Postrigor Myofibrils

Bovine semitendinosus muscles were sampled immediately after death, after 24 hr postmortem with storage at 2 degrees , 16 degrees , or 37 degrees C, and after 312 hr postmortem with storage at 2 degrees and 16 degrees C. A biopsy technique was used to prevent shortening during glutaraldehyde fixation. Postfixation in osmium tetroxide was followed by embedding in an Epon-Araldite mixture. Bovine muscle was supercontracted after 24 hr storage at 27deg; but was only slightly contracted after storage at 16 degrees for 24 hr. Muscle held at 37 degrees for 24 hr was slightly less supercontracted than the 2 degrees muscle. Striking similarities existed between muscles stored at 16 degrees and at 2 degrees C for 312 hr. Both were slightly shortened with narrowed I bands and an area of increased density, probably due to overlap of thin filaments in the middle of the A band. Postmortem shortening was accompanied by banding-pattern changes similar to those predicted for contracting muscle by Huxley and Hanson's sliding filament model. Treatment of myofibrils with 0.05% trypsin resulted in a rapid loss of Z lines and, in supercontracted myofibrils, caused a return of the banding pattern of resting muscle.